Electronically scanned sonars are utilized to detect the location of a target. Such sonars are mounted, for example, on buoys dropped by a plane and serve to transmit information on the location of a target, e.g a submarine, which emits sound waves or reflects echoes.
Electronically scanned sonars characteristically employ a cylindrical transducer array with a vertical axis over whose periphery are placed uniformly spaced hydrophones. Monitoring of the space surrounding the sonar is accomplished by establishing listening channels employing a delay line and by electronically scanning these channels of which each corresponds to a given bearing. As an example, such a sonar may have 28 rows of hydrophones spaced uniformly around an axis to form 56 listening channels corresponding to 56 bearings. Two channels are formed between every two hydrophones, the channels having positions symmetrical in relation to the line bisecting the angle defined by the two hydrophones.
If a sound is generated in the direction of a given channel, the resulting sound waves picked up by the different hydrophones have differences in phase or delay determined by the respective position of the channel and the hydrophones. The electric signals emitted by the hydrophones exhibit the same delays between them. Each hydrophone is connected to the input of a delay line which corrects the phase differences between the different signals and brings them all into phase. Consequently, if a target is located in the direction of a channel, there will be received, at the output of the delay line, an electrical signal which will be the sum of all the electric signals emitted by the different hydrophones. On the other hand, if the target is located in another direction, the phase shifts between signals emitted by the different hydrophones are no longer identical with the phase shifts introduced by the delay line and the sum of signals obtained at the output of the delay line will have a lower amplitude.
The channels and the hydrophones occupy respective positions which are repeated by rotation around the vertical axis constituting an axis of symmetry. The different channels can thus be scanned at a given rate by a "circular permutation" of the hydrophones connected to each of the terminals of the delay line.
In presently existing sonars, electronic scanning is accomplished by connecting to each of the terminals of the delay line, a multiplexer provided with as many inputs as the transducer array has hydrophones and a single output connected to such terminals. The multiplexer is of known design and is an electronic switching device with several inputs and only one output. Each input is permanently connected to a line receiving an electric signal. The multiplexer has an address file which receives pulses that successively switches the different inputs to the single output. For example, the different inputs can be energized by circular permutation of each one in turn at each new inpulse. The lines leading to the inputs can be energized in any given sequence and, the step of the permutation may be simple or multiple. Scanning devices having as many multiplexers as pickups are expensive and cumbersome.